1. Field of the Invention
This invention relates to an optical scanning system as employed in a laser printer and, more particularly, to an optical scanning system having a first light receiving means for controlling timing of scan and a second light receiving means for controlling the amount of a scanning beam.
2. Description of the Prior Art
An optical scanning system used in a laser printer causes the transverse scan of an image plane (hereinafter referred to as a main scan) by repetitively deflecting a light beam at high speed using a light deflector such as a polygon mirror and, at the same time, a vertical scan of the image plane (hereinafter referred to as a sub scan) by moving the image plane in the direction substantially normal to the main scan direction, thereby realizing a formation of image.
A laser printer of such a type requires very precise positioning of light beam in repetitive deflection in order to insure the quality of the image formed. To meet this demand, light receiving elements are disposed at the outside of both edges of an effective scan field so as to detect the timing of scan, whereby the scan timing is controlled in response to the detected timing.
Variations in the energy of the light beam during the scan across the image plane may sometimes result in an image inconstant in density and, hence, it is necessary to control the output of the light source by monitoring the amount of light beam in order to eliminate such variations in density. To this end, there is also provided another light receiving element, other than the elements for controlling the scan timing at a constant, so as to detect the amount of light beam, whereby the output of the light source is controlled in response to the detected amount of light beam.
Since the foregoing light receiving elements occupy predetermined physical dimensions, the necessary range to be scanned will actually be increased when the above two types of light receiving elements are aligned at the outside of the effective scan field in the scan direction. This results in a wider scan angle, a polygon mirror that will be increased in size if it is employed as the light deflector, and an optical scanning system that will be increased in size if the optical scanning system, having an f.theta. lens, is interposed between the light deflector and the image plane.
Since the optical path, which contributes to the formation of an image, and the optical path directed to the light receiving elements should be separated from each other, it is difficult to dispose the aforementioned light receiving elements in proximity to an image plane in terms of mechanical design, and, hence, the range of actual scan is increased.